1. Field of the Invention
The present invention relates to a communication system. More particularly, the present invention relates to a frame synchronization apparatus and a method based on differential correlation.
2. Description of Related Art
A communication system (for example, a digital video broadcasting-satellite (DVB-S) or a second generation system of digital video broadcasting-satellite (DVB-S2)) can use different frame types. A frame synchronization apparatus of the communication system is used for detecting a position of a physical layer frame header (PLHeader), so as to facilitate operations of a follow-up frequency offset correction and a forward error correction (FEC) decoder. Different to the DVB-S, a frame length of the DVB-S2 is longer, and a signal to noise ratio (SNR) is lower. Taking a frame type with pilot quaternary phase shift keying (QPSK) as an example, a frame length thereof is 33282 symbols, and a SNR thereof is −2.35 dB (for ¼ code rate). Therefore, it is important to use the DVB-S2 frame structure to quickly lock the position of the PLHeader.
For example, according to an article “Frame Synchronization and Pilot Structure for Second Generation DVB via Satellite” (International Journal of Satellite Communications and Networking, 2004; 22:319-339) authored by Sun Jian and Lee et al., a plurality of correlation values are calculated in search windows, and maximum correlation values are obtained from the correlation values. Assuming a length of a T-th search window WK,T of a K-th frame type is LK, according to such conventional technique, a sync word is used to perform a differential detection to each of the symbols, so as to obtain a maximum index IK,T from the correlation values of the search window WK,T. Such method is repeated to obtain the maximum indexes IK,T, IK,T+1, IK,T+2, . . . of the correlation values of a plurality of consecutive search windows WK,T, WK,T+1, WK,T+2, . . . . If IK,T=IK,T+1=IK,T+2= . . . , it can be determined that the corresponding position of each of the indexes IK,T, IK,T+1, IK,T+2 is the header position, i.e. the position of the PLHeader. However, in the conventional technique, the pilot information is not used, and a chance of miss-detection is excessively high in case of a low noise, which may lead to a longer synchronization time.
According to a U.S. Pat. No. 7,308,064, steps of “sync word correlation” and “multi-step correlation test” are used to search the position of the PLHeader. In the step of “sync word correlation”, as long as the correlation value of the PLHeader is greater than a threshold Γ1, the step “multi-step correlation test” is executed. In the step “multi-step correlation test”, if a pilot correlation value is greater than a threshold Γ2, a number of successful detection I is accumulated by 1 (i.e. I=I+1), and when I=PN, the synchronization is determined to be successful. However, in the step of “sync word correlation”, the determination performed by using the threshold can cause a false-alarm in case of a low SNR. In the step of “multi-step correlation test”, determination is performed to each of the pilots, though a large PN can cause the miss-detection and a small PN can cause the false-alarm. Moreover, in such conventional technique, a spectrum inversion phenomenon in a satellite channel is not considered, so that the step of “multi-step correlation test” can be failed in case of the spectrum inversion phenomenon. In addition, a non-pilot frame is not considered in the conventional technique.